Today, most of the display units used on computer terminals and the like output analog RGB video signals to a color display terminal. In keeping with the recent trend toward higher levels of display resolution, the display units utilize a horizontal deflection frequency of 90 kHz and a video signal bandwidth of about 150 MHz. This often leads to such problems as signal losses and unwanted radiated emission during transmission of the video signals from the computer proper to a display unit located away from it.
Conventional attempts to solve the above problems illustratively involve having the computer transmit low-frequency digital video data unmodified (i.e., without converting the video signals to analog signals) to the display unit.
One conventional technique of the above kind is disclosed in Japanese Patent Laid-Open No. 233779/1986. The disclosed technique involves retrieving digital video data from the video memory of the computer and outputting the data in parallel signal format to the display unit. In turn, the display unit converts the received data from digital to analog format. The resulting analog video signals are used to provide a video display.
The standard resolution for the grade of ordinary personal computer displays is illustratively 640 by 480 dots (640 horizontal dots and 480 vertical lines). Suppose that the RGB video signals are given eight-bit gradation each (i.e., 16.67 million color options per dot). In that case, the required rate of digital video data transfer from the computer to the display unit receives the drawing commands and audio commands, and is as high as 55 megabytes per second. The high resolution for the grade of workstation displays is illustratively 1280 by 1024 dots. With 256 color options assigned per dot, the necessary rate of digital video data transfer is about 80 megabytes per second. Today's trend is for the level of resolution to go up further. The number of color options per dot is approaching 16.67 million, so that the resolution is almost high enough to permit display of natural pictures on the display unit.
As described, where it is desired to have the display unit display pictures sent from the computer, the amount of digital video data to be transferred from the computer to the display unit is enormous. If the conventional technique outlined above is used to transmit digital video data in eight-bit parallel format, a data transfer clock frequency of about 55 MHz is needed for the standard resolution and about 80 MHz for the high resolution. One result of this is the diminished effectiveness in reducing the unwanted radiated emission. In practice, with the data transfer rate as high as is conventionally proposed, it is difficult to implement data transfer setups of adequate reliability free of dropped bits or noise intrusions. Furthermore, when the width of data to be transmitted is raised from eight bits to 16, 32, etc., the data transfer clock rate may be lowered but more and more cables are needed to perform digital video data transfer. Since the bulky cables are cumbersome to handle around equipment, it becomes increasingly impractical to locate the computer proper far away from the display unit.